Crystalline thermoplastic polybutylene terephthalate (PBT) resins have been used extensively as an engineering plastic in various fields owing to their excellent mechanical and electrical properties, as well as their physical and chemical characteristics. Thus, thermoplastic PBT resins have been used to form molded parts in various fields including automotive, electrical and electronic end-use applications.
PBT resins have typically not been employed as a feedstock for blow-molding operations since the intermediate hollow parison that is formed during the blow-molding process is undesirably "drawn-down" due to the poor melt tension that conventional PBT resin exhibits. Thus, hollow blow-molded articles have typically not been produced from PBT resins. Instead, PBT resins are conventionally thought of as an injection-moldable resin from which hollow injection-molded articles may be produced.
Prior attempts at forming hollow articles of PBT resins by blow-molding techniques have included increasing the molecular weight of the PBT resin so as to obtain a corresponding increase in melt strength (melt tension) and melt viscosity, since greater molecular weight typically results in a greater melt strength for a given resin. However, merely increasing the molecular weight of PBT resin to thereby increase its melt viscosity does not achieve the needed melt strength sufficient to employ PBT resins in blow-molding processes. Furthermore, the addition of inorganic filler materials has also been attempted as a means to increase the melt viscosity and melt strength of PBT resins sufficiently to allow blow-molding of hollow articles. Again, however, simply adding an inorganic filler to PBT resins (even those having an increased molecular weight) does not achieve the necessary melt strength properties necessary to allow the resin to be processed by blow molding techniques.
What has been needed in this art therefore, is a PBT resin composition which exhibits sufficient melt tension characteristics so that it could be employed economically in blow-molding processes. It is towards satisfying such a need that the present invention is directed.
Broadly, the present invention relates to novel blow-moldable PBT resins and to blow-molding processes for producing hollow articles of such PBT resins. More specifically, the present invention relates to blow-moldable PBT resins comprised of a PBT base polymer and an effective amount of between 0.01 to 5 parts by weight (based on 100 parts by weight of the PBT base polymer) of at least one organosilane compound. Preferred organosilane compounds are alkoxysilane compounds including vinylalkoxysilanes, epoxyalkoxysilanes, aminoalkoxysilanes, allylalkoxysilanes and mercaptoalkoxysilanes. One or more of these compounds are melt-blended with the PBT base polymer prior to blow molding.
Optionally, the PBT resin compositions of this invention may include a minor (but effective) amount of an olefinic copolymer. The preferred olefinic copolymer that may optionally be incorporated in the PBT resin compositions of this invention include copolymers of an .alpha.-olefin with an .alpha.,.beta.-unsaturated glycidyl ester. This olefinic copolymer may further be grafted with a vinyl copolymer to form a branched or crosslinked structure that is effective to improve the melt tension of the PBT resin composition and thereby enhance it blow-moldability.
Other aspects and advantages of this invention will become apparent after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.